1. Field of the Invention
This invention relates to purification of exhaust gases from the distillation of oil shale, and more particularly to cleaning distillation gases from oil shale by using electrostatic precipitators.
2. Background of the Invention
In the distillation retorting of oil shale, depending on the particle size of the shale which is distilled and the method of distillation used, a greater or lesser amount of dust will be carried out of the distillation reactor together with the distillation gas and be present in the oil after subsequent cooling and condensation. The finer the shale is ground, the greater the amount of dust. Removing dust from the oil is very expensive and reduces the yield of oil.
Removal of dust from distillation gases by means of cyclone separators is known. However, this procedure has the disadvantage that fine dust can be only partially removed. When electrostatic precipitation is used to remove dust from combustion gases, the difference between the corona initiation voltage and the sparkover voltage decreases as the gas temperature increases. Furthermore, it has been found in the case of a number of dust-containing gases, e.g. exhaust gas from the preheating of finely ground cement raw mixes with furnace flue gases, that they cannot be cleaned in the temperature range 310.degree. C. to 350.degree. C. by electrostatic precipitation, but they can be cleaned at higher temperatures, e.g. 400.degree. C. to 420.degree. C. However, this is not the case with the reducing gases derived from distillation in the temperature range above 300.degree. C.
A hot reducing gas produced by burning natural gas with an insufficient supply of air will contain the following components: nitrogen, carbon dioxide, carbon monoxide, water vapor, hydrogen, and methane, as well as smaller amounts of higher hydrocarbons. Such a gas cannot be cleaned by electrostatic precipitation in the temperature range which has to be used, because the difference between the corona initiation voltage and the sparkover voltage is too small.
Hence, a need has continued to exist for a method of removing dust from the distillation gases produced when oil shale is distilled.